Microwave high power short pulse shaper



y 1969 H. w. COOPER ETAL 3,458,817

MICROWAVE HIGH POWER SHORT PULSE SHAVER Filed Feb. 15. 1967 2 Sheets-$heet 1 THYRATRON wAvEGuIDE SWITCH-. a 34 33') 4| ELEcTRoNIcALLY |ZL+ r swITcII VARIABLE DELAY TRIGGER H GEE kI H OR D TRANSMITTER TRIGGER TRIGGER PRE-PULSE A A ANDDE CURRENT H g ITRIDDE/ l FIRED ELECTRON DENsITY DEcAY PERIOD ELEcTRoN DENSITY c I F|G.2. "1 4 TRIGGER DELAY TIME DETECTED ENVELOPE I IMEGAWATT sLIcE OF PULSE 0F INPUT POWER I TO BE GENERATED I I (SEE CURVE E) I I I I I 40 DETECTED ENVELOPE OF E OF SHORT PULSE OUTPUT I FLAT PART 50 DB l I (saw SPIKE PEAK W DELAY TIME 1I- Tws FIRING TIME wITNEssEs: INvENToRs Herbert W. Cooper and ATTORNEY July 29 1969 w, COOPER ET AL 3,458,817

MICROWAVE HIGH POWER SHORT PULSE SHAPER Filed Feb. 13, 1967 2 Sheets$heet 2 MICROWAVE GENERATOR r D TWS n TWS TWS Tws 3 41 -,.n4so 53 42041 N x p Hz-56o vmso TRIGGER (PREPULSER) ouTPu PULSE mom MG mpur PULSE TO rws4s OUTPUT PULSE mom ANTENNA 4? INPUT PULSE TO TWS 52 OUTPUT PULSE mom ANTENNA s3 3,458,817 MICRGWAVE HTGH POWER SHORT PULSE SHAPER Herbert W. Cooper, Hyattsville, and Harry Goldie, Randallstown, Md., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Feb. 13, 1967, Ser. No. 615,475

Int. Cl. H04b 1/04; H01p 1/14; G01s 7/28 US. Cl. 325180 6 Claims ABSTRACT OF THE DISCLOSURE The waveguide section constituting the control element of a gaseous triode, commonly called a t'hyratron waveguide switch, is incorporated into a waveguide transmission line through which microwave energy is being propagated. The plasma created by the current arc between the cathode and anode of the triode is used to derive extremely short pulses from a longer pulse which is only partially propagated through the control-electrode waveguide section. The control of the amplitude of any individual pulse over a wide dynamic range, such as 40 to 50 db, can be eifected by adjustment of a time differential between the low energy video pulse and the high power microwave pulse. This time differential causes the wavefront to be incident on the TWS at any desired plasma electron density since the plasma slowly decays after the arc current ceases. This time ditference may be effected at an electronic rate which results in an amplitude modulation impressed on the transmitted pulse.

In applicants patent application Serial No. 383,639, now Patent Number 3,323,003 filed July 20, 1964, there is described the thyratron waveguide switch which is an essential component of the present invention. In brief, the thyratron waveguide switch, herein otherwise referred to as TWS, is an externally controlled gas-type switch for use in high speed, high power microwave applications where extremely low loss and wide bandwidths are required. The device is not RF-activated and therefore a source of triggering pulses is necessary to actuate it. The electrical characteristics are such that it is relatively insensitive to ambient temperature variations. In the TWS the control electrode is a section of microwave guide which is adapted to be inserted in the usual waveguide transmission line in which it is desired to control the propagation of microwave energy. The section of waveguide is sealed to the envelope that encloses the anode and cathode and the waveguide section is provided with pressure windows to complete the envelope that retains the hydrogen gas atmosphere around the electrodes. When triggering pulses are applied between the control electrode and the cathode a plasma is generated in the region of the cathode and this causes the tube to fire and a current are to form between the cathode and the anode. This are creates a high density plasma that extends across the section of the waveguide serving as the control electrode and this plasma serves as an RF barrier to provide attenuation to microwave energy.

In applicants copending patent application there is disclosed and claimed a circuit for operating a TWS in a grounded grid configuration as an alternative to the grounded cathode configuration shown herein.

This invention relates to a control circuit, and particularly to one for producing timing sequences for driver circuits for microwave generators. More particularly, the invention relates to such a driver circuit which is capable of deriving extremely short microwave pulses from a longer pulse at very high power levels in rectangular 3,458,817 Patented July 29, 1969 waveguide. Additionally this invention relates to a scheme for high level amplitude modulation at electronic rates.

Heretofore, microwave short pulse generators in the nanosecond rang have not been capable of producing peak power outputs of more than several watts, in general, in the neighborhood of 5 watts depending on frequency band. The devices of the prior art used for accomplishing this result are varactor or PIN diodes; that is, diodes that can be triggered into the conducting condition as to act as a switch. The present invention makes it possible to raise the power level of the microwave output from microwav short pulse generators in the nanosecond range to at least 300 kilowatts and up to the multi-megawatt range.

The device used to shape the short microwave pulses from a longer pulse is a gaseous triode in the form of a thyratron waveguide switch. The waveguide section of the switch which serves as the control grid for the TWS is inserted in the waveguide transmission line between the source of the microwave energy and the antenna. As is well understood in the art, the plasma which is formed inside of the control-electrode waveguide section constitutes an RF barrier which blocks the propagation of the microwave energy when the gaseous triode is in the conducting condition. In accordance with this invention, suitable delaying and timing circuits are provided between the trigger pulse generator which actuates the gaseous triode and the device which actuates the microwave longpulse transmitter. The gaseous triode is fired first to establish the plasma in th waveguide transmission line and thereafter the pulse forming network is discharged to initiate the microwave transmitter energy at such time that as the plasma in the triode decays, a portion of the microwave burst will pass through the transmission line to the antenna. The plasma will not attenuate the leading edge of the wavefront because of the formative time lag associated with a gas discharge. The remainder of the transmitters pulse will be absorbed in the dummy load connected to the circulator.

From the above it will be apparent that the purpose of this invention is to provide an extremely short high power microwave pulse. Other and further objects will be apparent from the following description when considered in connection with the accompanying drawings, in which:

FIGURE 1 is a circuit diagram of the present invention;

FIG. 2 is a graphical illustration of the timing sequence of the video and the RF pulses when using the single gaseous triode circuitry of FIG. 1;

FIG. 3 is a modified form of the invention using more than one gaseous triode as a pulse shaping device in order to derive a series of short pulses from one long pulse; and

FIG. 4 is a graphical illustration of the timing sequence of the video and RF pulses from a plurality of antennas as shown in FIG. 3.

According to the embodiment of the invention illustrated in FIG. 1, a source of microwave energy is illustrated as being a magnetron 10, the output of which is connected to circulator 11 through input arm 12. The circulator has an arm 13 connected to an absorptive load 14 and another arm 16, which is part of waveguide transmission line to the antenna 18. A waveguide section 19, which serves also as a control-electrode of the TWS thyratron waveguide switch 21, of which it is a part, is interposed and connected between the waveguide arm 16 and the waveguide transmission line 17.

As is known in the art, the waveguide section serving as the control electrode of TWS device is sealed to the envelope 21a of the device which has the usual cathode 22 and conventional anode 23. The waveguide section 19 has pressure windows 24 and 26 which are provided with suitable irises to permit the passage of microwave energy 3 and also serve to complete the envelope to contain the gaseous atmosphere around the electrodes. The TWS controls the time at which propagation of microwave energy is allowed through its waveguide section-control electrode 19 by reason of the plasma indicated at 25 formed between the cathode and the anode when the TWS is fired.

Because of the characteristics of this device, it has been found that by a proper timing sequence of the video pulse and the RF pulses a selected portion in the early part of the time period of a longer pulse from the microwave generator can be selected to produce extremely short microwave pulses in the neighborhood of 20 nanoseconds or less from a longer pulse at high peak levels which may vary from 0.3 to megawatts in rectangular waveguide.

To this end, the TWS 21 may be operated as a grounded cathode device, as illustrated in FIG. 1, with a suitable trigger pulse generator 31 being provided to supply a positive triggering pulse 41 over the conductor 32 to the waveguide section control electrode 19 for controlling the conduction condition of the device. Since the accomplishment of the end results of this invention depend upon the proper timing sequence between the firing of the TWS 21,

and the incidence of the microwave power through the control electrode-section 19, means is provided to utilize the same trigger pulse that fires the TWS 21 to control the operation of the magnetron 10. This is accomplished by supplying the trigger pulse from the generator 31 through a suitable delay unit 33 to a conventional discharge switch 34 which controls the discharge of the pulse forming network 36 through the primary of the pulse transformer 37. As is conventional, the anode 23 of the TWS 2'1 and the pulse forming network 36 may be connected to a common source of DC potential represented by the terminal 38.

In the operation of the embodiment of FIG. 1, a low energy video pulse 41 from the trigger generator 31 pretriggers the TWS 21 causing plasma to be created in the region adjacent the cathode 22. Due to the anode voltage, plasma 25 is formed within the waveguide section 19, causing a high current are to form between the cathode 22 and the anode 23 with the current reaching very high values, such as 10 to amperes. The degree of electron density Within the plasma is then a function of the anode current. After the discharge are current ceases, which may be in the neighborhood of 0.1 microseconds (as shown in FIG.2(B)), the free electron density decays slowly toward zero which is shown in FIG. 2(C). In the upper curve (A) of FIG. 2, the pre-triggered pulse is indicated as the first in the timing sequence of the video and RF pulses. In the next curve 2(B) the anode current is indicated and the electron density is indicated in curve 2(C).

The RF power of the magnetron 10 is delayed with reference to the video pulse. The delay is such that the microwave transmitter burst is incident at the input arm 12 of the circulator 11 during the plasma decay time. The microwave pulse is indicated in curve (D) of FIG. 2 and it shows that the incidence of the microwave energy begins just about the instant that the electron density reaches its maximum value. Since the radio frequency breakdown power depends on the electron density within the gaseous triode, the peak RF output power will vary with the delay time between the video pulse and the leading edge of the RF pulse. With the appropriate time setting of delay unit 33 and switch 34 this time delay may be varied at an electronic rate. Thus, the instantaneous width may be varied. The detected envelope of the short pulse output through the arm '16 of the circulator to the waveguide section 19 to the antenna 18 is illustrated graphically in curve (E) of FIG. 2.

Immediately after the RF breakdown interval the attenuation of the plasma in the waveguide section 19 between the cathode and the anode is sufiiciently high so that the remainder of the RF pulse is at least 50 db below the peak level of the triangular output pulse. It is to be noted that for a constant input RF power level, the output pulse 40 has a triangular shape and approximately constant width at the half power point while its peak level may change from 1 kilowatt to 10 megawatts within milliseconds.

It will be seen that the RF barrier in the TWS 21, produced by the plasma 25 effectively slices, in a timewise direction, a very small portion of the microwave pulse from the magnetron 10 and permits this slice of pulse to reach the antenna 18. Therefore, the energy in the remainder of the pulse must go somewhere, and it is delivered through the arm 13 of the circulator to the absorptive load 14. Since there is a very high proportion of the energy in the microwave pulse which is not utilized when a single TWS switch is used, as in FIG. 1, more eflicient pulse shaping can be accomplished by creating a series of short RF pulses from the long RF input pulse. This embodiment of the invention is illustrated in FIG. 3 where the trigger delays are adjusted to fire the different triodes, corresponding to the TWS 21, at the appropriate time intervals.

In this arrangemet, it will be apparent that instead of the microwave energy from the microwave generator MG being reflected from the plasma RF barrier in the first TWS 42 into a dummy load, such as the absorptive load 14 of FIG. 1, arm 43 of the first circulator 44, corresponding to the arm 13 of FIG. 1, is connected through a second TWS 46 to a second antenna 47. Arm 48 of circulator 44 is connected to the input arm of a second circulator 49. Arm 51 of the latter is connected through a third TWS 52 to a third antenna 53. Arm 54 of the circulator 49 is connected through a fourth TWS 56 to a fourth antenna 57. Likewise, arm N of circulator 48 may be connected to an additional circulator, not shown, and so on. This could be extended to a practical limit so that a slice of the microwave long pulse would be radiated from each of several antennas.

In this embodiment the trigger pulse generator 61} generates, on output terminals, groups of pulses 42a, 46a, 52a, 56a and Na which are supplied in the sequence indicated above to the respective thyratron waveguide switches 42, 46, 52, 56, etc.

In the operation of this embodiment, assume that the pulse 42a has just triggered TWS 42 and as its plasma decays a portion of the microwave burst from the microwave generator MG will pass to antenna 41 and be radiated. The remainder of the microwave burst will circulate counterclockwise in circulator 44 and a second portion of the original microwave burst will pass into arm 43. The proper timing sequence of pulse 46a supplied to TWS 46 will permit a similar portion of the microwave burst to pass to the antenna 47 and the remainder of microwave energy will pass on to the arm 48 and into the second circulator 49 where similar action takes place. A graphical representation of this action is indicated in FIG. 4.

It will be seen from the foregoing description that the present invention provides means for selectively deriving one or more very short high peak power microwave pulses from a single high power microwave burst of substantial length.

We claim as our invention:

1. A microwave pulse modulation system comprising a microwave generator forming a source of pulsed microwave energy, a circulator connected to said source having at least two output arms, an antenna, a waveguide transmission line connected between one of said arms and said antenna, a thyratron waveguide switch in said transmission line for selectively attenuating the propagation of microwave energy in said line, said other arm being connected to means for accepting microwave energy reflected from said waveguide switch, means for triggering said thyratron waveguide switch and said pulsed microwave source including delay means for triggering said microwave generator at a selected time interval after the triggering of said waveguide switch to permit only a selected portion of the microwave pulse from said generator to reach said antenna.

2. The combination as set forth in claim 1 in which said thyratron waveguide switch includes a perforated waveguide section serving as the control electrode forms a part of the transmission line from said circulator to said antenna.

3. The combination as set forth in claim 2, in which said means for accepting microwave energy reflected from said waveguide switch is a second circulator for guiding microwave energy to a second antenna.

4. The combination as set forth in claim 2, in which said means for accepting microwave energy reflected from said waveguide switch is an absorptive dummy load.

5. The combination as set forth in claim 1, in which said means for accepting microwave energy reflected from said waveguide switch is an absorptive dummy load.

3,085,239 4/1963 Hoover 343-171 XR 3,226,647 12/1965 Shirrnan 333-31 XR 3,276,019 9/1966 Fackler 325-180 XR 3,023,380 2/1962 Hill 333-13 3,396,388 8/1968 Goldie 333-13 XR RICHARD MURRAY, Primary Examiner CARL R. VON HELLENS, Assistant Examiner U.S. Cl. X.R. 

